1. Field of Invention
The present invention relates to a method for fabricating an integrated circuit (IC). More particularly, the present invention relates to a method for fabricating a gate oxide layer.
2. Description of Related Art
Currently, in the semiconductor process, the integration of the IC has become higher as the size of the field effect transistor (FET) becomes smaller. When the size of FET is smaller than 0.1 .mu.m, the thickness of the gate oxide layer is smaller than 20 .ANG.. However, the gate oxide layer is made from 6.about.7 layers of SiO.sub.2 molecules equivalent to a thickness of about 20 .ANG.. So, electrons may tunnel through the gate SiO.sub.2 layer and cause leakage. Since the gate SiO.sub.2 layer is thinner, it may be tunneled through by the dopant ions. Thus, this causes electrical problems for the device. As the size of the FET is usually limited by the thickness of the gate SiO.sub.2 layer, many dielectrics having high dielectric constants are currently being developed to solve the problems introduced by the gate SiO.sub.2 layer mentioned above. These dielectrics include Ta.sub.2 O.sub.5, Pb(Zr, Ti)O.sub.3 (i.e. PZT), and (Ba, Sr)TiO.sub.3 (i.e. BST), wherein Ta.sub.2 O.sub.5 has a dielectric constant of about 20-25, the BST has a dielectric constant of about 20-60, and the PZT has a dielectric constant of about 600-1000.
Among the three types of dielectrics mentioned above, the process for Ta.sub.2 O.sub.5 is most compatible with the current process. As Ta.sub.2 O.sub.5 has to undergo recrystallization after its formation, in order to purify and strengthen its structure, the oxygen employed during the recrystallization may oxidize the silicon substrate to form a SiO.sub.2 layer between the silicon substrate and the Ta.sub.2 O.sub.5 layer. Since the SiO.sub.2 layer has a lower dielectric constant, the Ta.sub.2 O.sub.5 layer formed by the conventional method is only able to obtain a dielectric constant of about 15, instead of a basic value of 25. Thus, the Ta.sub.2 O.sub.5 layer formed by the conventional method cannot satisfy the need for further integration in future devices, and substitute dielectrics, such as BST and PZT which have high dielectric constants, should be used.
However, when BST and PZT are used as dielectric layers, current leakage problems may easily occur since these dielectrics have exceedingly high dielectric constants. So, many considerations have to be made for such processes. The compatibility problem of the whole process may also be encountered with BST and PZT serving as dielectric layers. Therefore, it is not easy to integrate the method mentioned above with the current process.